This invention relates generally to compositions and methods for the formation of protective, corrosion-inhibiting rinses and seals for use to impart additional corrosion resistance to structural materials without the use of chromium in the hexavalent oxidation state. More particularly, this invention relates to non-toxic, corrosion-protective rinses and seals for metal phosphating, anodizing, and “black oxiding” processes based on trivalent (or tetravalent) cobalt and methods of making and using the same.
Metals like aluminum, zinc, titanium, iron, cadmium, tin, indium, manganese, beryllium, magnesium, niobium, tantalum, zirconium, lead, cobalt, copper, and silver, their alloys, or items plated with these metals, require protection from corrosion due to their low oxidation-reduction (redox) potentials or ease of oxide formation. These metal alloys have many uses that range from architectural adornments to protective coatings themselves to automotive, structural aerospace, and electronic components, to name a few. The unalloyed metals typically form an outer layer of natural oxide: a “passive film” that serves to protect them and reduce their overall rate of corrosion. However, the corrosion protection offered by the naturally formed oxide layer on certain alloys of these metals is not complete and corrosion will eventually occur unless some form of additional corrosion protection is used. Thus, for example, steels are typically “phosphated” to provide an impermeable coating that not only resists corrosive attack, but also provides a paint base. Additionally, architectural and structural aluminum are frequently “anodized” to form an impermeable oxide film for the same reasons.
Inhibiting the initiation, growth, and extent of corrosion is a significant part of component and systems design for the successful long-term use of metal objects. Uniform physical performance and safety margins of a part, a component, or an entire system can be compromised by corrosion.
One method to enhance the corrosion resistance of these alloys is through the use of a chemically- or electrolytically-generated coating such as an anodized coating (typically on aluminum), a phosphate coating (typically on electrogalvanized or bare steel), or a black oxide coating (for high strength bearing and tool steels). The metal is exposed to a compound that chemically alters the surface (in phosphating and black oxiding) or an electric current (in anodizing) and forms a coating that provides some corrosion resistance by forming a barrier film. The morphology and possibly the chemistry of the anodic coating or phosphate coating can allow for the formation of a strong bond with subsequently-applied paint systems. An anodic coating is usually applied via immersion in an electrolytic cell. A phosphating or black oxide solution may be applied by immersion, by spray, or by manual means.
These coatings frequently exhibit “flaws” such as pores, pinholes, or thin portions in the coating after formation and do not contain any inherent means to “repair” these coating breaches. The application of a second solution is necessary to fill the pores in the coating and deposit compounds that will act as long-term corrosion protective species. These “second solutions” are termed “rinses” or “seals” in the corrosion literature. The term “rinse” is typically used for the second solution applied to phosphating and black oxide coatings, whereas the term “seal” usually refers to the second solution applied to anodic coatings. These rinses and seals are typically applied via spray techniques, but immersion, fogging, and wiping are also accepted practices.
Hexavalent chromium has traditionally been the active corrosion-inhibiting agent used in rinses and seals for the formation of protective coatings for iron, electrogalvanized iron, aluminum, zinc, magnesium, titanium, cadmium, tin, indium, manganese, and their alloys. Niobium, tantalum, zirconium, beryllium, lead, cobalt, copper, and silver may also be treated with hexavalent chromium rinses and seals for special applications. The three main coating processes that use these rinses and seals are 1) the phosphating process for steel and galvanized steel products, 2) the anodization process for a host of structural metals, and 3) the black oxide process for high-strength steel and iron used for bearing materials. Table 1 illustrates the processes that typically utilize a final chrome “rinse” or “seal” to impart additional corrosion protection to a given substrate material.
TABLE 1Current Rinse and Seal Processes Using Hexavalent ChromiumComments/Government/ProcessExamplesSubstrate MetalsASTM/Mil SpecsRinses for zincUsed as a paint baseZinc-coated steel,MIL-P-50002phosphating onon all automotivezinc, or bare steelDoD-P-16232steel, steel products,bodies, also for someare usual substrates.MIL-HDBK-205and nonferrouscoil and sheet stock.Also for aluminum,SAE-AMS2481alloysUsed as a lubricatingmagnesium, copper,QQ-P-416layer on tooling dies.titanium, cadmium,and silver in lesscommonapplications.Seals for anodizedUsed extensively forAluminum andMIL-A-8625aluminum includingarchitectural andaluminum alloysSAE-AMS2470sulfuric, chromic,decorativeASTM B580oxalic, boric,applications,ASTM D1730sulfonated organicadhesive bonding,AA46-78acids, citric, andsiding, etc. Alsophosphoric acidused as a paint base.anodizingRinses for ironUsed as a paint baseSteel and iron alloysTT-C-490phosphating on bareon coil coatings forMIL-HDBK-205steelsgeneral applianceSAE-AMS2481and sidingQQ-P-416applications.Different from Znand Mn phosphating.Rinses forUsed solely as a solidMostly bare steel.MIL-P-50002manganeselubricant, not as aCan also be used onDoD-P-16232phosphating on steelpaint base. Usedhigh-strength copperMIL-HDBK-205and steel alloys, alsoextensively onalloys.SAE-AMS2481on nonferrous alloysbearing materials.Rinses for “blackUsed solely as a solidMostly bare steel.MIL-C-13924oxide” and otherlubricant, not as aCan also be used onMIL-C-46110oxide lubricatingpaint base. Usedhigh-Strength copperSAE-AMS2485layersextensively onalloys.bearing materials.Seals for anodizedUsed as a paint andMagnesium andMIL-M-45202magnesiumadhesive base.magnesium alloysASTM D1732including sulfuric,SAE-AMS2475chromic, oxalic,MIL-C-13335boric, sulfonatedorganic acids, citric,and phosphoric acidanodizingSeals for anodizedUsed as a paint andTitanium andSAE-AS4194titanium includingadhesive base.titanium alloysSAE AMS-2488sulfuric, chromic,oxalic, boric, citric,hydrofluoric, andphosphoric acidanodizingSeals for anodizedUsed as a paint andZinc and zinc alloysMIL-A-81801zinc includingadhesive base.sulfuric, chromic,oxalic, boric,sulfonated organicacids, citric, andphosphoric acidanodizingSeals for anodizedUsed as a paint andIron, steel, and steelQQ-P-35steel includingadhesive base.alloyssulfuric, chromic,oxalic, boric, andphosphoric acidanodizingSeals for anodizedUsed for a number ofCopper, cadmium,QQ-P-416copper, cadmium,applications,silver, tantalum,silver, tantalum,principally as a paintniobium, zirconium,lead, cobalt,and adhesive base.tin, indium,niobium, zirconium,For example,manganese and theirtin, indium, andniobium andalloysmanganesetantalum capacitors,including sulfuric,cadmium plate, silverchromic, oxalic,solder, and zirconiumboric, sulfonatedfor nuclearorganic acids, citric,applications.and phosphoric acidanodizing
As shown in Table 1 above, there are three “generic” phosphating processes for steel and steel alloys—zinc, manganese, and iron phosphating. Differences in the coating solutions result in different chemistries and physical attributes in the formed coatings. For example, zinc phosphating is used primarily on galvanized steel sheet, and results in an ideal surface morphology for paint adhesion if the crystals are small in size, and as a solid lubricant for larger size crystals. Manganese phosphating, however, results in a hard, lubricious coating that has no use as a paint base, but exhibits excellent characteristics as a solid lubricant. Manganese phosphating coatings are rarely subjected to a post-chrome rinse, because the corrosion resistance of these coatings is of lesser concern. Iron phosphating is also used as a paint and adhesive base, and always receives post-treatments for corrosion protection.
Similar differences are also noted in anodizing processes. Anodizing processes involve the application of an electric potential under a variety of acidic conditions to the substrate to be coated. Sulfuric acid is the conventional anodizing acid used to form hard oxide films on aluminum, although other anodization solutions have specialized applications. For example, phosphoric acid may be used for adhesive bonding applications on aluminum. Oxalic acid anodization results in a harder, denser coating with higher corrosion resistance than sulfuric acid anodization and is used more often in Europe. Boric acid anodization is used frequently for electronic capacitors although citric and tartaric acid anodization can be used for the same application. Anodization with sulfonated organic acids (such as sulfosalicylic or sulfophthalic acids) is used to impart color during the anodization process. Chromic acid anodization is used on parts with complex shapes where final sealing or rinsing is not possible. Other acids, including hydrofluoric acid, have been used for special applications or in proprietary formulations. Those skilled in the anodization art know that there exist a wide variety of anodizing processes due to the multitude of substrate metals, anodizing acids, applied voltages, and final applications.
Finally, “black oxide” coatings are applied to high strength steels and copper-containing alloys to impart a lubricious coating. The difference between “black oxide” coatings and other lubricious coating processes (such as manganese phosphating) is that “black oxide” coatings are applied under caustic, elevated temperature conditions. For example, a concentrated sodium hydroxide solution is raised to its boiling point and the substrate metal is then immersed in this solution. This results in the formation of a lubricious coating of magnetite/ferrite on the surface of steel alloys.
Other coating processes that result in coatings with no inherent self-healing characteristics have also been enhanced through the use of hexavalent chromium rinses and seals. Carbonate coatings on metals such as zinc, iron, magnesium, and especially copper have been described in the early literature as providing some degree of corrosion protection. These coatings can be further enhanced through the use of hexavalent chromium rinses to deposit inhibiting compounds to self-heal coating breaches. Other oxide, phosphate, oxalate, silicate, aluminate, borate or polymeric coatings, or combinations thereof, can also be enhanced via hexavalent chromium rinses and seals.
For each of these three generic coating processes (phosphating, anodizing, and black oxiding), a second, subsequent chemical treatment is often applied. The nature of this second treatment is dependent upon the desired final characteristics of the metal piece. For phosphating and black oxiding processes, this second treatment is usually a rinse of hexavalent chromium, to impart additional corrosion protection to the coating. For anodizing processes, the second treatment can impart a number of useful attributes to the work piece. This second “sealing” process for anodized coatings can include: 1) pure boiling water (to plug the pores with a hydrated alumina composition); 2) silicates (to plug the pores with a silicate composition); 3) dyes or metal-dye complexes (to impart color to the anodic coating); 4) metal salts followed by cathodic reduction (to color the coating via the formation of metals or metal sulfides in the pores); 5) lubricating additives such as molybdenum disulfide or dispersions of polytetrafluoroethylene (to fill the pores with a lubricious additive); and 6) hexavalent chromium seals to fill the pores with chromate species. It is noteworthy that the only one of these six generic sealing processes that results in a coating with self-healing characteristics is the hexavalent chromium seals. The other sealing processes for anodic coatings may temporarily increase the corrosion resistance of the coating by plugging the pores in the oxide coating (e.g., with hydrated alumina or silicate), but the coating does not retain any corrosion-inhibitive species.
The various coating processes to which the art described in this invention is applicable are shown in Table 1 above. The frequent use of hexavalent chrome to “rinse” or “seal” the coating (phosphate, anodic, or black oxide) formed in the first unit operation of the process, to impart additional corrosion resistance, connects them. These solutions are usually simple formulations consisting of nothing more than dissolved chromium trioxide, chromate, or dichromate. These formulations are usually applied by spraying, although immersion, fogging, or even wiping may also be used.
Sometimes these hexavalent chromium rinse or sealing formulations will contain other constituents. Some formulations include minor concentrations of fluorides. These fluorides act to “etch back” the coating formed in the first unit operation (e.g., phosphate, anodic, or black oxide), thus further facilitating the deposition of corrosion-inhibiting species. Rinsing solutions for phosphate solutions are frequently observed to include phosphoric acid in addition to hexavalent chromium in order to reduce staining of the phosphate coating by the hexavalent chromium. These hexavalent chromium rinse or sealing solutions can also contain other constituents, such as ferricyanides or molybdates. The presence of these other constituents is significant in light of the chemistry developed and presented in this invention.
Significant efforts have been made to replace chromium with other metals for corrosion-inhibiting applications due to toxicity, environmental, and regulatory concerns. Cobalt is one non-toxic, non-regulated metal that has been considered as a chromium replacement. Cobalt (like chromium) exhibits more than one oxidation state (Co+2 and Co+3). In addition, the oxidation-reduction potential of the Co+3-Co+2 couple is comparable to the Cr+6-Cr+3 couple. For example, in acid solution:Co+3+eCo+2+1.92 VCr+6+3eCr+3+1.36 VAccordingly, a number of processes have been reported in the literature, which make use of cobalt in rinsing or sealing bath solutions, generally to provide coloring of the coated alloys. However, the coatings formed by these processes provide only limited corrosion protection and do not approach the benefit derived from the use of hexavalent chromium. None of the prior art recognizes the importance of trivalent (or tetravalent) cobalt for corrosion protection, nor the need to “valence stabilize” trivalent cobalt to ensure its long-term stability. The use of cobalt in the prior art is primarily as a coloring agent for anodic coatings, although there is some reference to its use as a rinse for phosphate coatings. The use of cobalt in rinses for black oxide coatings has heretofore been unrecognized.
The use of film-forming substances, such as polymers, silicates, sol-gel, etc., which have no inherent oxidizing character in sealing or rinsing coating solutions, has been described in the literature. The film formers may enhance short-term corrosion resistance by functioning as a barrier layer. Barrier layers lacking an active corrosion inhibitor have been demonstrated to be capable of inhibiting corrosion as long as the barrier is not breached, as by a scratch or other flaw. Film formers can actually enhance corrosion on a surface after failure due to the well known effects of crevice corrosion.
1) Rinses for Phosphate Coatings
U.S. Pat. No. 4,673,445 to Tuttle, Jr., et al. describes the use of a 175° F. post-treatment for phosphate coatings that contains cobalt, a tin (II) compound [stannous], and tartaric acid. Given the proper pH conditions, it may be possible to form a stannate-stabilized cobalt complex from this solution. However, no conditions are described that would result in oxidation of the cobalt to the trivalent or tetravalent oxidation state. Stannous compounds are mild reducing agents so it is extremely unlikely that Co+3 or Co+4 could be formed from these solutions in the absence of any oxidizing species.
European Patent No. EP 0 486 778 B1 to McMillen, et al. describes the use of rinsing solutions that contain an amino compound (amino acid or amino alcohol) and a transition metal compound. However, the preferred group IIIB and IVB transition metal and rare earth metal compounds described are zirconium, titanium, hafnium, cerium, and mixtures thereof.
2) Seals for Anodic Coatings
Cobalt has primarily been described as a coloring agent for anodized coatings, under a variety of different processing conditions. These include:
a) Electrolysis in the absence of valence stabilizer compounds once an anodic coating is formed. Coloring is frequently accomplished by immersing the work piece into a separate, cobalt-containing solution and then electrolyzing. However, these solutions typically do not contain materials that can function as valence stabilizers, nor are subsequent treatments with compounds that can function as valence stabilizers described. Additionally, electrolysis is performed under conditions that reduce the cobalt-to-cobalt metal, cobalt-containing alloys, or reduced cobalt compounds such as sulfides. This process involves connecting the anodized work piece to the electrolytic cell so that it functions as a cathode to reduce the cobalt. An example of this is described in European Patent No. EP 0 368 470 B1 to Fern, et al. A “pore-filling”metal (cobalt is a described example) is deposited into the pores using an a.c. or modified a.c. deposition. Long-term corrosion resistance will be decreased due to the formation of galvanic couples between the anodized substrate metal and the pore-filling metal, while a temporary increase in corrosion resistance may be expected due to the filling of the pores. The use of trivalent or tetravalent cobalt to provide long-term corrosion protection is not described in these patents.
b) Use of cobalt-dye complexes to color anodic coatings has been used since the 1950s with metal complexes of azo dyes, sulfonic acids, amino acids, aromatic carboxylic acids, and other organic compounds. Many of these compounds can be used as “valence stabilizers” for trivalent cobalt. The oxidation state of the cobalt in the described coloring materials is always divalent. Divalent cobalt provides no redox-based corrosion-inhibiting protection. The use of cobalt in corrosion-inhibiting seals for anodic coatings has been described less often than for coloring. None of these compositions describe the use of trivalent (or tetravalent) cobalt as the inhibitor species, nor the use of “valence stabilizers” to provide long-term corrosion-resistance.
Accordingly, the need remains for improved rinses and seals which have an effectiveness, ease of application, and performance comparable to coatings formed with hexavalent chromium and which do so without the use of toxic or currently regulated materials.